The present invention relates to a method and apparatus for producing blown hollow glass bodies having an optical inner wall structure. More specifically, the invention is directed toward a procedure wherein a heated glass blank having a structured inner surface is blown in a blank mold which is formed with a smooth inner surface. The glass body is produced by pressing a tapered plunger against the heated glass blank which has been provided within the mold, the plunger having a tapered configuration which narrows toward the direction in which the plunger is introduced into the mold. The plunger is provided with elevations and/or recesses upon its exterior which form a molding pattern for the glass blank. The surfaces of the molding pattern are arranged to extend at least partially angularly relative to the plunger axis. In the process of the present invention the blank is finish-blown in a finishing mold.
In the prior art it has been known that certain optical effects can be achieved in hollow glass bodies if the inner wall of such bodies is formed with a relief structure while the outer wall thereof is smooth. Known methods for the production of such hollow bodies include, for example, a method described in German Pat. No. 2,147,349 wherein a hollow blank having an inner wall structure is first produced in a blank mold from a glass drop by means of a plunger having an outer structure. The glass is subsequently brought into a final form by suction or pressure blow deformation. The application of this method, however, encounters difficulties because of the fact that the plunger, which must be introduced into a glass mass which is still in a plastic state in order to form the inner relief structure, must subsequently be withdrawn from the formed glass after a pressing operation. In order to permit this approach to be effectively applied, the plunger may only have a molding pattern on its outer surface which is comprised of surfaces which are helically extending or parallel to the longitudinal axis of the plunger.
With a molding pattern involving axially parallel surfaces, a plunger which is tapered in the direction in which it is pressed into the glass mass may be removed after the pressing operation by withdrawing the plunger axially from the blank mold containing the plastic glass mass. However, such an operation is not possible where the molding pattern on the plunger contains surfaces, elevations or recesses which extend angularly relative to the plunger axis. This is due to the fact that the respective recesses or elevations formed in the glass blank would be sheared off during the axial withdrawal of the plunger by the boundary surfaces of the molding pattern formed upon the plunger exterior.
Such boundary surfaces of the elevations and/or recesses of the molding pattern extending angularly to the plunger axis are only possible if they are arranged in a helical configuration, with the plunger being turned out from the blank mold corresponding to the pitch of the helix.
In any case, the elevations and/or recesses must be steady; that is, they must not involve interruptions since such interruptions will lead to sawtooth interlocking of the glass material and of the plunger thereby preventing the plunger from being smoothly withdrawn from the mold. Thus, certain limits are created with regard to the type of inner wall structure which may be produced. For example, it is not possible to produce with known methods a relief structure on the inner wall of a glass body where the relief surfaces extend transversely to the direction of motion of the plunger.
Another disadvantage of known prior art methods involves the fact that the production of hollow glass bodies having helical elevations and/or depressions requires a rather elaborate rotary drive feed mechanism which must be adapted exactly to the helical course of the plunger. It will be obvious that this will involve considerable structural complexity requiring substantial production efforts and high costs.
Blanks for the production of hollow glass bodies having any type of desired inner wall structure could, heretofore, be produced only with plungers which include a plurality of compressed air channels. In this manner, a blank may be widened by means of compressed air conducted through the channels thus enabling the plunger to be removed therefrom. The disadvantages of such a method, however, involved the fact that the compressed air channels give rise to a considerable increase in the cost of the plunger which thereby requires additional pressure tight compressed air structures of elements. Furthermore, the blank mold must be removed before the widening operation and the plastic blank must be supported by another mold during such widening.
The present invention is intended to provide a method for avoiding the aforementioned problems and overcoming the disadvantages discussed above. The invention permits the production of hollow glass bodies whose inner wall structure consists of elevations and/or depressions which include surfaces which extend at least partly angularly to the axis of the plunger. The solution offered by the present invention involves a method wherein the plunger is axially removed from the blank mold or blank after it has been axially pressed thereinto. The invention is essentially based upon the concept of designing the plunger so that no plunger surfaces exist which will be behind the glass material of the blank, taken in the direction in which the pressing operation is performed. In this manner, the plunger may be removed from the blank easily and without damage. The invention provides a plunger whose molding surfaces either detach themselves immediately from the inner surface of the blank or slide, as in the case of axially parallel plunger surfaces, at first along the opposite inner surface of the blank.